1. Field of the Invention
The present invention relates to an optical writing and reading method and apparatus for writing data to or reading data from a recording medium by optical means. More particularly, it relates to an optical writing and reading method and an optical writing and reading apparatus providing an effective discrimination process for preventing an over write onto a medium on which data has been already written.
2. Description of the Related Art
An optical disc unit is well known as an optical reading and writing apparatus. An optical disc unit acting as an external large capacity storage unit for use with a data processing unit, has a high density recording availability ten times or more higher than a conventionally used magnetic disc unit. Also, long term data storage is possible with the optical disc unit because the writing of data to the optical disc unit is carried out by, for example, opening a hole (pit) by thermal reaction caused by irradiating a light beam onto an optical disc medium. That is, by strongly focusing a laser beam on the recording material, a pit (hole) is thermally formed through the recording film (Note: there are other methods of modifying the recording film) so that data is written therein.
Accordingly, if new data is written onto the recording film in which pits (holes) are already formed, both the new data and the already written data are destroyed. Therefore, in one of the conventional methods to prevent an over writing to an already written region, after data is written, a mark indicating that data has been written (hereinafter this mark is referred to as a write mark) is made on the region immediately before the region where the data was written. Therefore, two rotations of the disc medium are necessary to write the data and the write mark.
Because of the increased frequency of use of such an optical writing and reading apparatus, there is a strong demand for the provision of a method by which over writing will be prevented and an efficient and accurate discrimination can be made of whether or not the data write-in region has data written thereon.